The goal of this observational study is to observe the correlation between dynamic changes in the Doppler-based renal resistive index (RI) and the occurrence of persistent acute kidney injury (AKI) in critically ill patients. The main questions it aims to answer are:

• Are dynamic changes of RI within 24 or 48 hours associated with the prognosis of AKI (NO AKI, transient AKI, persistent AKI)?
• what are the influencing factors of changes in RI? Participants will measure values of RI in patients at admission (0 hour), after 24 hours and after 48 hours, respectively, and collect characters such as age, sex, vital signs, Apache II, diagnosis, renal function, applied drugs, fluid, 72-hour renal function and renal replacement, followed renal replacement and the mortality rate at 28 days.

NO AKI is defined as No AKI occurred within 72 hour after ICU admission; Transient AKI is defined as AKI with recovery, which refers to the reduction in AKI stage ( (i.e., a decrease of serum creatinine, reversal of oliguria without diuretic therapy, and without need for renal replacement therapy) occurring with 72 hour after ICU admission; Persistent AKI is defined as AKI without recovery occurring with 72 hour after ICU admission.

Inclusion Criteria:

• Patients admitted to the ICU within 24 h of admission.

Exclusion Criteria:

• Chronic kidney disease stage III-V;
• History of kidney transplantation;
• Previously known renal artery stenosis or renal vein thrombosis；
• Cardiac arrhythmia precluding renal Doppler measurement;
• Expected hospital stay < 72 h；
• Pregnant patients
• Last, patients with hospital stays < 72 h, since they could not be classified as having transient or persistent AKI according to our definitions, were secondarily excluded.

All the critically ill patients